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GPS Applications in Space

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Space Situational Awareness 2015: GPS Applications in Space James J. Miller, Deputy Director Policy & Strategic Communications Division May 13, 2015 GPS Extends the Reach of NASA Networks to Enable New Space Ops, Science, and Exploration Apps GPS Relative Navigation is used for Rendezvous to ISS GPS PNT Services Enable: • Attitude Determination: Use of GPS enables some missions to meet their attitude determination requirements, such as ISS • Real-time On-Board Navigation: Enables new methods of spaceflight ops such as rendezvous & docking, station- keeping, precision formation flying, and GEO satellite servicing • Earth Sciences: GPS used as a remote sensing tool supports atmospheric and ionospheric sciences, geodesy, and geodynamics -- from monitoring sea levels and ice melt to measuring the gravity field ESA ATV 1st mission JAXA’s HTV 1st mission Commercial Cargo Resupply to ISS in 2008 to ISS in 2009 (Space-X & Cygnus), 2012+ 2 Growing GPS Uses in Space: Space Operations & Science • NASA strategic navigation requirements for science and 20-Year Worldwide Space Mission space ops continue to grow, especially as higher Projections by Orbit Type* precisions are needed for more complex operations in all space domains 1% 5% Low Earth Orbit • Nearly 60%* of projected worldwide space missions 27% Medium Earth Orbit over the next 20 years will operate in LEO 59% GeoSynchronous Orbit – That is, inside the Terrestrial Service Volume (TSV) 8% Highly Elliptical Orbit Cislunar / Interplanetary • An additional 35%* of these space missions that will operate at higher altitudes will remain at or below GEO – That is, inside the GPS/GNSS Space Service Volume (SSV) Highly Elliptical Orbits**: • In summary, approximately 95% of projected Example: NASA MMS 4- worldwide space missions over the next 20 years will satellite constellation. operate within the GPS service envelope (**) Apogee above GEO/GSO (*) Source: Aerospace America, American Institute of Aeronautics and Astronautics (AIAA), Dec. 2007 Medium Earth Orbit: Orbital Transfers: LEO-to- GNSS Constellations, GSO, cislunar transfer orbit, etc., transplanetary injection, etc. GeoSynchronous: 3 Communication Satellites, etc., GPS Space Service Volume (SSV) Concept Partnership with DoD Space Service Volume (Medium Altitudes) • Four GNSS signals available simultaneously a majority of the time • GNSS signals over the limb of the earth become increasingly important with altitude • One-meter orbit accuracies Space Service Volume (High Altitudes) • Nearly all GNSS signals are received over the limb of the Earth • Periods when no signals are available • Signal levels will be weaker than those in Terrestrial Service Volume (TSV) • Positioning software uses orbital physics, and/or stable on-board oscillators, to achieve orbit accuracy of tens of meters 4 Space Service Volume: Using GPS Beyond LEO and up to GeoSynchronous Altitude • 3,000 to 8,000 km Medium Altitudes –Four GPS signals usually available simultaneously, however poor geometry & coverage gaps cause harm –1 meter accuracies still feasible, however space GPS receivers have more difficulty processing signals –GPS performance degrades with altitude due to geometry and classic near/far problem • 8,000 to 36,000 km High Altitudes –Users will experience periods when no GPS satellites are available – Point Positioning no longer available –Nearly all GPS signals received over limb of the Earth – High variability in signal strength and beam paths –Received power levels are weaker than those in TSV or MEO SSV – Side Lobe processing needed –Specially designed receivers will be capable of maintaining accuracies ranging from 10- 100 meters depending on receiver 5 sensitivity and local oscillator stability 5 Expanding the GPS Space Service Volume (SSV) into a multi-GNSS SSV • At least four GNSS satellites in line-of-sight are ≥ 4 GPS satellites in line-of-sight here needed for on-board real-time autonomous (surface to 3000 km) navigation – GPS currently provides this up to 3,000 km altitude – Enables better than 1-meter position accuracy in real-time • At GSO altitude, only one GPS satellite will be available at any given time. – GPS-only positioning at GSO is still possible with on- board processing, but only up to approx. 100-meter absolute position accuracy. – GPS + Galileo combined would enable 2-3 GNSS sats in-view at all times. – GPS + Galileo + GLONASS would enable at least 4 GNSS sats in-view at all times. – GPS + Galileo + GLONASS + Beidou would enable > 4 GNSS sats in view at all times. This provides best accuracy and, also, on-board integrity. Only 1-2 GPS ... but, if interoperable, • However, this requires: satellites in then GPS + Galileo + – Interoperability among these the GNSS line-of-sight GLONASS + Beidou provide constellations; and – Common definitions/specifications for the Space here (GSO) > 4 GNSS sats in line-of- Service Volume (3,000 km to GSO altitude) sight at GSO 6 Using GPS above the GPS Constellation: NASA MMS Mission – GSFC Team Info Magnetospheric Multi-Scale (MMS) Mission • Launched March 12, 2015 • Four spacecraft form a tetrahedron near apogee for performing magnetospheric science measurements (space weather) • Four spacecraft in highly eccentric orbits – Phase 1: 1.2 x 12 Earth Radii (Re) Orbit (7,600 km x 76,000 km) – Phase 2: Extends apogee to 25 Re (~150,000 km) MMS Navigator System • GPS enables onboard (autonomous) navigation and potentially autonomous station-keeping • The MMS Navigator system exceeded all of the team’s expectations, it has set the record for the highest GPS use in space • At the highest point of the MMS orbit Navigator set a record for the highest-ever reception of signals and onboard navigation solutions by an operational GPS receiver in space • At the lowest point of the MMS orbit Navigator set a record as the fastest operational GPS receiver in space, at velocities over 35,000 km/h 7 MMS Navigator System: Initial Observations • In the first month after launch, the MMS team began turning on and testing each instrument and deploying booms and antennas. – During this time, the team compared the Navigator system with ground tracking systems and found it to be even more accurate than expected – At the farthest point in its orbit, some 76,000 km from Earth, Navigator can determine the position of each spacecraft with an uncertainty of better than 15 meters – The receivers on MMS have turned out to be strong enough that they consistently track transmissions from eight to 12 GPS satellites – excellent performance when compared to pre-flight predictions of frequent drop outs during each orbit 8 Why is the Space Service Volume Important? SSV specifications are crucial for providing real-time GNSS navigation solutions in High Earth Orbit • Supports increased satellite autonomy for missions, lowering mission operations costs • Significantly improves vehicle navigation performance in these orbits • Enables new/enhanced capabilities and better performance for future missions, such as: Improved Weather Prediction using Space Weather Observations Astrophysics Observations Advanced Weather Satellites En-route Lunar Formation Flying & Closer Spacing of Satellites in 9 Navigation Support Constellation Missions Geostationary Arc Satellite Laser Ranging (SLR) on GPS III • Laser ranging to GNSS satellites enables the comparison of optical laser measurements with radiometric data, Satellite Orbit Laser Reflector Array identifying systemic errors on a Satellite • Post-processing this data allows for refining station coordinates, satellite orbits, and timing epochs • The refined data enables improved models and reference frames Laser Pulse • This results in higher PNT accuracies for all users, while enhancing interoperability amongst constellations • NASA Administrator Bolden worked with Air Force Gen Laser Ranging Shelton & Gen Kehler to approve Laser Reflector Arrays Station (LRAs) onboard GPS III • Plans are now underway to deploy LRAs on GPS III starting Laser pulse (start) with Space Vehicle 9 for launch in the 2020 timeframe Photon return (stop) Measurement of round trip time of laser pulse Station coordinate and satellite orbit 10 determination relative to Earth’s center Augmenting GPS in Space with TASS TDRSS Augmentation Satellite Service (TASS) 1) User Acquires GPS signals GEO Relay 2) GDGPS Tracks GPS signals Satellite 3) GEO Satellite Relays Differential 4) Evolved TASS could Corrections to User incorporate: • GPS integrity information • Tracking satellite GPS Satellites information (health, ephemerides, maneuvers) • Space weather data • Solar flux data • Earth orientation parameters Space User • User-specific commands • PRN ranging code GDGPS Network 11 11 Closing Remarks • NASA and other space users increasingly rely on GPS/GNSS over an expanding range of orbital applications to serve Earth populations in countless ways • The United States will continue to work towards maintaining GPS as the “gold standard” as other international PNT constellations come online • NASA is proud to work with the USAF to contribute making GPS services more accessible, interoperable, precise, and robust for all appropriate users • GPS precision enables incredible science, which in turn allows NASA to use this science to improve GPS performance “On Target with GPS Video” www.youtube.com/watch?v=_zM79vSnD2M 12 .
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